In contrast to cationic redox dyes such as methylene blue, and xanthene dyes which undergo one electron redox reaction, leuco azine dyes of the present invention undergo a two electron redox reaction involving a quinonoid form, the azine dye form.
The preparation of certain azine dyes by oxidative coupling of phenothiazine or phenoxazine with carbon or nitrogen nucleophiles has been described by, inter alia, F. Kehram (Ann. Chem. 1902, 322, 1, 45) J. A. Van Allan et al (J. Org. Chem. 1969, 34, (6), 1691) and J. Daneke (Ann. Chem. 1970, 740, 52).
We have discovered a range of new azine dyes, and their corresponding leuco or reduced forms, which are useful in various imaging systems.